Modal Stinger Kit
to apply force to a structure under test
The desired force input points on many test structures lie at a considerable distance above ground level. For tests on such items, it becomes difficult and expensive to use Fixed Body Mode support structures. For such applications, the Free Body Mode can be employed. In this mode the shaker body is used as the reaction mass by suspending the shaker from an overhead support. Besides using a force transducer a very convenient measure of the load force is the axial acceleration of the shaker body. This allows simplification of the test system instrumentation, in that force can be measured with an accelerometer system which is identical to that used to measure the structural response.
In the Fixed Body Mode, the shaker body is fixed and the armature is attached to the test structure. So shaker force is delivered directly in the test structure. In this mode, force delivered to the test structure can be measured by inserting a force transducer in the thrust linkage between the armature and test structure. Alternatively, with the armature/body suspension bands removed, shaker current can be used as a direct measure of the generated force. The generated force is approximately equal to the force delivered to a resonant test structure, because the armature mass is typically small compared to the modal mass of most large test structures.
Auxiliary table accessory units for both horizontal and vertical use employ the high load capability of the APS 113 armature guidance and suspension systems to provide long stroke tables for excitation of test loads. Each auxiliary table has a pattern of threads with helical inserts to mount the test load directly or a fi xture on the table. The mechanical input impedance at the base of a test load that is resonant in the operating frequency range can vary
signifi cantly. Thus, the acceleration response of the table and test item base will exhibit the familiar „peaks“ and „notches“ as frequency is varied. So it is recommended to observe the response of the structure using accelerometers.
Many large test structures having horizontal surfaces such as fl oors require vertical or horizontal force applied to these surfaces to generate resonant modes of vibration. The APS shakers may be used in a vertical or horizontal Free Armature Mode by resting the shaker body on the horizontal surface. The moving armature provides a reaction mass that allows for delivery of the shaker force via the shaker body to the surface (inertial shaker). Below a certain cross-frequency the shaker stroke limits the acceleration of the armature and thus the maximum force. Additional masses, e.g. APS 0112 or 4001, lower this frequency. The force applied to the structure can be measured by a force transducer or by measuring the acceleration of the moving masses.